Dichlorobutenes (3,4-dichlorobutene and 1,4-dichloro-2-butene) are intermediates in the industrial synthesis of chloroprene (2-chloro-1,3-butadiene). Chloroprene gives high molecular weight elastomer polymers or synthetic neoprene used in tires and tire products, rubber goods, footwear, adhesives, latex foams, and latex paints. Most chloroprene is polymerized to make polychloroprene, a synthetic rubber used in wire and cable covers, gaskets, automotive parts, adhesives, and other applications requiring chemical, oil, and weather resistance or high gum strength.
Chloroprene is produced from butadiene in three essential steps. First is chlorination to yield a mixture of dichlorobutene isomers, then isomerization of 1,4-dichloro-2-butene to the desired 3,4-isomer, and, lastly, caustic dehydrochlorination.
Commercial production of chlorobutenes from butadiene is based almost entirely on vapor phase chlorination. The crude chlorination products are condensed from excess butadiene which is then recycled to the reactor. Systems for refining the crude product vary depending on the degree of integration with the subsequent steps, and whether the 1,4-isomer is desired for separate uses or for complete isomerization to the 3,4-isomer. The streams to be separated are (1) low boiling impurities, mainly 1- and 2-chlorobutadiene, (2) purified 3,4-dichlorobutene, (3) 1,4-dichloro-2-butene and (4) high boiling tri- and tetrachlorides. During this purification process, the feedstock must be heated at temperatures from 120.degree. to 160.degree. C. which results in high polymerization and fouling of the purification equipment.
Due to the chlorine content and temperatures employed in the processing system, corrosion of the processing system can occur. This corrosion will cause iron and other metal particles to be present in the dichlorobutene feedstock in amounts as high as 10 parts per million. Iron is a transition metal and its presence in the dichlorobutene feedstock can contribute to the unwanted polymerization of the dichlorobutene.